/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Adds input pre-processor if unique.
True if added to list of input pre-processors, false if not
bool RegisterInputPreProcessor ( [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< class [IInputProcessor](API\Runtime\Slate\Framework\Application\IInputProcessor) > InputProcessor, const int32 Index )